Network TX/RX fairness is not honored by USB stack

[Hans Petter Selasky]

On Saturday 02 October 2010 02:11:00 Pyun YongHyeon wrote:
> Hi,
> 
> I don't know how long it had been there but it seems current USB
> stack does not honor fairness of TX/RX on USB ethernet controller.
> Unidirectional performance test(UDP) or most-unidirectional
> performance(TCP) test works well without problems. However if heavy
> TX/RX traffic hits controller at the same time either TX or RX is
> not served at all. I'm under the impression that whenever TX work
> is done it seems USB reschedules next pending TX again instead of
> processing RX such that RX is starved to death. This can be easily
> reproduced on two hosts with the netperf performance test.
> Whenever both hosts send tiny UDP datagrams to the other host
> either TX or RX packet counters are not increasing until the end
> of the UDP torture test. The number of EHCI interrupt is about 8K/s
> while test is in progress so I think it reached its maximum
> processing limit. After netperf testing, it can still process TX/RX
> packets even though it dropped too many RX packets. But these
> dropped packets are not counted so netstat(1) shows 0 dropped
> frames even though it lost millions of packets.
> 
> Hans, do you have any idea what's going on here?
> You can use the following netperf command on both hosts after
> running netserver.
> %netperf -c -H ip_addr_of_other_host -tUDP_STREAM -l 300 -- -m 1
> 
> Another odd thing I noticed is number of interrupts does not go
> down to 0 after the testing. It constantly generates 1k/s
> interrupts after that.

Maybe we are triggering a bug. Can you enable USB debugging to figure out what 
data lengths are transmitted or received.

USB EHCI uses round robin, so this is either USB device problem or a test-
program software failure.

Check the CPU usage of the host computer during the test. Do you see anything?

> The only way I stop that interrupts was to
> down the ue0 interface with "ifconfig ue0 down" command.

--HPS